Auxetic Foam Cell Structure
add Cube
use addon “Destructability Editor” or official “Cell Fracture” to crack cube into Voronoi shards





Join all shards Ctrl+J
in edit mode - “Remove Doubles” (W)
select all outer faces and choose delete "Edges"
than delete “Only Faces”





add Modifiers - Smooth (here with Factor 2)





Switch to "Edge" select
Select "Random" from menu like 20-50% and delete selection





Subdivide (W) like 2-4


This created net you can render by adding "Skin" modifier, but its very heavy for calculation, so I used here to add Mball to the scene and add Particle system to the net with "Render" > "Object" > "Mball".






Probably would be good to know the purpose of your needs. In some cases could be enough and probably looking better simple Volumetric material with texture type Voronoi or Musgrave.



Under Properties > Render > Geometry > Volume Sampling > Step size decrease value to something like 0.0100. In Blender 2.79 are some changes on Volume Shader, I don't know how to colorised foam to get the same result.